Polyether polyols which are suitable for the preparation of polyurethanes can be obtained by means of various preparation methods. On one hand, base-catalysed addition of alkylene oxides to H-functional starter compounds, and on the other hand the use of double metal cyanide compounds as catalysts (“DMC catalysts”) are of significance for the addition of alkylene oxides to H-functional starter compounds on an industrial scale. The (Lewis) acid-catalysed addition of alkylene oxides to suitable starter compounds is of secondary importance.
Undesirable secondary reactions increase considerably with increasing molar mass of the polymer under alkali metal hydroxide catalysis. In particular, mention should be made here of the isomerisation of propylene oxide to allyl alcohol, which at high equivalent weights (or low hydroxyl values) results in a high proportion of monofunctional polyether species in the reaction mixture. The monofunctional polyether molecules have an adverse effect on the full curing behaviour and the profile of physical properties of polyurethane systems.
The use of DMC catalysts has made it possible to push ahead with the addition of alkylene oxides, in particular propylene oxide, to H-functional starter compounds down to very low hydroxyl values, without the above-mentioned isomerisation of propylene oxide to allyl alcohol occurring to a significant extent. Highly active DMC catalysts, which are described e.g. in U.S. Pat. No. 5,470,813, EP-A 700 949, EP-A 743 093, EP-A 761 708, WO-A 97/40086, WO-A 98/16310 and WO-A 00/47649, in addition have exceptionally high activity and permit polyether polyol preparation at very low catalyst concentrations (25 ppm or less), so that it is no longer necessary to separate off the catalyst from the finished product. Typical examples are the highly active DMC catalysts described in EP-A 700 949, which in addition to a double metal cyanide compound (e.g. zinc hexacyanocobaltate(III)) and an organic complexing ligand (e.g. tert-butanol) also comprise a polyether polyol having a number-average molar mass of greater than 500 g/mol.
One characteristic of DMC catalysts is their pronounced sensitivity to high concentrations of hydroxyl groups, which are caused for example by large amounts of starters such as ethylene glycol, propylene glycol, glycerol, trimethylol propane, sorbitol or sucrose, and polar impurities of the reaction mixture. The DMC catalysts cannot then be converted into the polymerisation-active form during the reaction initiation phase, or alkylene oxide addition reactions which are already running may come to a halt due to the continuous supply of high concentrations of hydroxyl groups and polar impurities. Impurities may for example be water, compounds with a high number of hydroxyl groups which are in close proximity, such as carbohydrates and carbohydrate derivatives, or compounds with basic groups such as for example amines. Substances with carbonyl groups which are in close proximity, or carbonyl groups which are adjacent to hydroxyl groups also have an adverse effect on the catalyst activity. In order nevertheless to be able to subject starters having high concentrations of OH groups, or starters having impurities which are to be regarded as catalyst poisons to DMC-catalysed alkylene oxide addition reactions, the hydroxyl group concentration has to be lowered or the catalyst poisons rendered harmless, respectively. For this purpose, first prepolymers can be prepared from these starter compounds by means of base catalysis, which prepolymers then after working-up are converted into the desired alkylene oxide addition products of high molar mass by means of DMC catalysis. What is disadvantageous with this procedure is that the prepolymer obtained by means of base catalysis has to be worked up very carefully in order to rule out deactivation of the DMC catalyst by basic catalyst traces introduced by the prepolymer.
This disadvantage can be overcome by the method of continuous metering of starter which is disclosed in WO-A 97/29146. In this case, critical starter compounds are not initially introduced into the reactor, but continuously supplied to the reactor during the reaction in addition to the alkylene oxides. Prepolymers can be initially introduced as starting medium for the reaction in this method, and also it is possible to use small amounts of the product to be prepared itself as starting medium. The necessity of first having to prepare prepolymers which are suitable for further alkylene oxide additions separately is thus dispensed with.
Likewise, polyether polyols can be prepared fully continuously without working-up in accordance with a method as described in WO-A 98/03571. In this case, in addition to one or more alkylene oxides and one or more starters, the DMC catalyst is also supplied continuously to the reactor or to a reactor system under alkoxylation conditions, and the product is removed continuously from the reactor or the reactor system after a pre-selectable average residence time.
Both the method of continuous starter admetering and the fully continuous polyether polyol preparation method have the disadvantage that polyethers with block structures, in particular those with short internal blocks, can be prepared only with great difficulty: in the case of the method of continuous starter admetering, the admetering of starter has to be concluded already before the end of the metering of the first alkylene oxide block in order to obtain products with homogeneously distributed block lengths. This is difficult in particular when internal blocks with block equivalent molar masses of 53 Da to 350 Da are desired, since it then becomes necessary to increase the ratio of starter to alkylene oxide in the admetered educt stream such that again there is the risk of attaining critical concentrations of hydroxyl groups and polar impurities. In such cases, the catalysts increasingly lose activity during the starter admetering phase, which manifests itself e.g. by an increase in pressure in the reactor as a result of an increasing concentration of free alkylene oxide. In the fully continuous polyether polyol preparation method, costly series of reactors and hold-up sections with a continuous throughflow have to be installed for products with block structures. Both the continuous starter admetering method and the fully continuous method are furthermore only poorly suited for converting high-melting starter compounds or starter compounds which decompose below the melting point, such as for example sugar, sorbitol or pentaerythritol, into long-chain polyols without working-up. Such starters have to be metered via expensively heated metering sections or in solution.
“Equivalent molar mass” is to be understood to mean the total mass of the material comprising active hydrogen atoms divided by the number of active hydrogen atoms. In the case of materials containing hydroxyl groups, it is calculated by the following formula:equivalent molar mass=56,100/hydroxyl value [mg KOH/g]
The hydroxyl value can be determined e.g. titrimetrically in accordance with the specifications of DIN 53240 or spectroscopically by means of NIR.
EP-A 0 090 445 claims the addition of catalyst “promoters” in order to increase the activity of DMC catalysts of an older generation. Such “promoters” are salts of at least divalent metal cations and metal-free anions, and/or metal-free acids. The “promoters” are added separately to the catalyst/starter mixture. It is emphasised that the absence of alkali metal salts is essential, since these reduce the activity of DMC catalysts. Given this background, the present invention is particularly surprising.
EP-A 1 400 281 claims salt-containing, in particular alkali metal halide-containing, DMC catalysts which result in polyethers with a reduced content of high-molecular impurities. In the present invention, starters containing potassium chloride however prove completely unsuitable, since no catalyst activation was observed.
EP-A 1 577 334 claims starters which are preferably acidified with phosphoric acid in DMC-catalysed alkylene oxide addition processes with continuous starter admetering, which result in increased catalyst life, if relatively short-chain polyethers are to be prepared by means of DMC catalysis, i.e. relatively high starter/alkylene oxide ratios are present during the metering phase. The addition of (acidic) salts is not mentioned. Work carried out in the context of the present invention shows that phosphate-containing starter compounds prevent activation of DMC catalysts.
WO-A 99/14258 likewise claims acidified starters in DMC-catalysed alkylene oxide addition processes with continuous starter admetering. Again, phosphoric acid is emphasised as particularly preferred acid. Salts of sulfuric acid are not mentioned.
U.S. Pat. No. 6,642,423 claims a method for obtaining polyethers with ethylene oxide-containing internal blocks. These can be obtained in one stage directly by DMC-catalysed ethylene oxide addition to low-molecular starter compounds such as glycerol, followed by a propylene oxide block or a block rich in propylene oxide. The method does not utilise the advantageous effect of the presence of a sulfuric acid salt on the suppression of the formation of high-molecular impurities, and is furthermore very expensive, since DMC catalysts in contact with low-molecular starter compounds such as glycerol can be activated only in very high concentrations.
EP-A 1 528 073 claims the two-stage preparation of typical long-chain polyols having an ethylene oxide end block in a reactor. The residual alkalinity resulting from the preceding batch in each case is removed before or during the metering of the starter and the DMC catalyst for the following batch by addition of an acid which forms a salt which is soluble in the long-chain polyol having an ethylene oxide end block. Generally only an alkylbenzenesulfonic acid is suitable for this, since the polyol remnants remaining in the reactor have high equivalent molar masses and are not capable of dissolving salts of purely inorganic acids. What is disadvantageous when using alkylbenzenesulfonic acids are the high costs, which are caused firstly by the high prices of acid, and secondly by the relatively high molar masses of the acids. Furthermore, the claimed process always requires a working-up step such as distillation/filtration or ion exchange, in which the large, conventionally obtained amounts of salt are removed.
WO-A 2006/094979 claims a simplified method for the preparation of DMC catalysts in which the cyanometallate acid is prepared in situ due to the presence of strong mineral acids during the catalyst precipitation. The catalysts thus prepared are conventional DMC catalysts, with which alone no one-pot method without working-up for the preparation of long-chain block copolyethers having internal blocks with block equivalent molar masses of 53 Da to 350 Da can be produced.
In WO-A 01/53381, combinations of Lewis or Brønsted acids and DMC catalysts are used, which are said to result in shortened induction periods upon starting-up the alkylene oxide addition reaction. Synergistic effects on polyether quality which are obtainable by the combination of certain acids/acidic salts with DMC catalysts have not been worked out; rather, the analytical data of the resulting polyether polyols, in particular the elevated contents of primary hydroxyl groups, simply indicate acid-catalysed or DMC-catalysed alkylene oxide addition reactions which take place in parallel.
EP-A 1 073 689 claims the preparation of polyol precursors with hydroxyl values from 100 to 150 mg KOH/g under Lewis-acid conditions, followed by a DMC-catalysed propylene oxide addition step. Separation of the Lewis-acid catalysts, substantially perfluoroalkylsulfonic acid salts of lanthanides, from the precursor before the DMC step does not take place. This is thus a one-pot method without working-up, in which however the striking tendency, described in EP-A 0 855 417, of the Lewis-acid catalysts to form volatile by-products and the high costs thereof have to be classed as disadvantageous.
WO-A 2007/082596 teaches the preparation of DMC catalysts modified with alkali or ammonium salts, which are distinguished by increased activities. It is not possible to carry out a method without working-up departing from low-molecular starter compounds with the method disclosed in WO-A 2007/082596. The positive effects of starters containing sulfuric acid salts with regard to the formation of high-molecular impurities cannot be attained in accordance with the teaching of WO 2007/082596.